Factors affecting growth and health of the fetus are of interest from both the practical and theoretical standpoints. The applicants propose to investigate several maternal factors in fetal and placental growth using a novel Peromyscus (deermouse) species hybrid system. In this system reciprocal hybrids exhibit exaggerated growth and mortality effects. This study will show the effects on embryonic growth of rendering mothers immune or tolerant to paternal antigens. The genetics of this sytem will be ascertained by a backcross analysis. The study will also show the relative significance of egg cytoplasm versus uterine environment on fetal growth by transfer of hybrid embryos to recipient mothers of opposite species. In addition the effects of somatotrophin on embryonic growth in these hybrids may be tested. These experiments should help clarify questions concerning immunological relationships between the mother and fetus, and may suggest means of modulating embryonic growth by immunological or hormonal manipulation.